a) Field of the Invention
The present invention is directed to the short coherence interferometry “z-scan” or depth scan for measuring the depth position of light-reemitting object structures in optical coherence interferometry (PCI=Partial Coherence Interferometry) and in optical coherence tomography (OCT) by matching the optical length of the reference arm of a two-beam interferometer to the optical length of the measurement arm by means of the interference occurring at the interferometer output.
b) Description of the Related Art
In optical PCI and OCT, depth positions of light-reemitting object structures are measured by means of short coherence interferometry by matching the optical length of the reference arm of a two-beam interferometer to the length of the measurement arm. In this connection, the object 1 forms a mirror of a two-beam interferometer, for example, a Michelson interferometer, as is indicated in FIG. 1. The interferometer is illuminated by a light beam 2 of short coherence length from a short coherence light source 3, for example, a superluminescent diode. The beam splitter 4 splits the illuminating light beam into a measurement beam and a reference beam. The portion of the light beam 2 reflected at the beam splitter 4 travels to the reference mirror 6 of the interferometer as a reference beam 5. The proportion of determination of the z-position is carried out with an accuracy given approximately by the coherence length
      l    c    ≅            λ      2        Δλ  of the light which is used; in this case, λ is the mean wavelength and Δλ is the wavelength bandwidth of the radiation that is used. In order to detect the x-coordinate in OCT, the object 1 is moved in x-direction or, as is indicated in dashed lines in FIG. 1, the measurement beam scans the x-coordinates at the object 1 by means of a rotating or oscillating rotating mirror 11.
There are a number of modifications of this basic method of OCT, some of which are described in the general overview by A. F. Fercher, “Optical Coherence Tomography”, J. Biomed. Opt. 1 (1996): 157–173. Common to all of these modifications is the need for light sources with spatially coherent light.
Spatially coherent light sources have the characteristic that their temporal coherence, and therefore the coherence length of the emitted light, is also relatively large, which is disadvantageous when used in PCI and OCT. At the present time, typical wavelength bandwidths of the superluminescent diodes used in PCI and OCT are 30 nm, for example. At a typical mean wavelength of 800 nm, this gives a coherence length of about 21 micrometers. However, this results in a rather poor accuracy for determining the z-position compared with other interferometry methods.
Conversely, light sources with short coherence length also have low spatial coherence. While there are also laser light sources with high spatial coherence and very short temporal coherence such as the Titanium-Sapphire laser, the latter is exorbitantly expensive and represents a complicated technology.
Light sources with especially short coherence length include transverse multimode lasers and transverse multimode light emitting diodes (LEDs) on the one hand and broadband spontaneously emitting light sources on the other hand. Further, transversely oscillating superluminescent diodes would also have an extremely short coherence length. Transversely oscillating superluminescent diodes of this kind are obtained when the active zone of these light sources has a thicker construction, which would also appreciably increase their output power (however, they are not yet commercially available due to a lack of possible applications). All of these light sources have only partial spatial coherence. Moreover, the spontaneously emitting light sources do not generate light based on induced emission like lasers, superluminescent diodes and LEDs. These light sources on the one hand and broadband spontaneously emitting light sources on the other hand. Further, transversely oscillating superluminescent diodes would also have an extremely short coherence length. Transversely oscillating superluminescent diodes of this kind are obtained when the active zone of these light sources has a thicker construction, which would also appreciably increase their output power (however, they are not yet commercially available due to a lack of possible applications). All of these light sources have only partial spatial coherence. Moreover, the spontaneously emitting light sources do not generate light based on induced emission like lasers, superluminescent diodes and LEDs. These light sources include incandescent lamps, arc lamps and gas discharge lamps. Halogen bulbs, for instance, emit light in a range of λ=500 nm to λ=1300 mm. Coherence lengths of lc=0.9 μm correspond to this. These light sources also have only partial spatial coherence. Their use in OCT would, for example, appreciably improve the attainable z-resolution on the one hand and, due to the simple handling and cheapness of these light sources, would also reduce the cost of PCI and OCT devices.